Blepharitis is a common eye condition characterized by chronic inflammation of the eyelid, usually where eyelashes grow, resulting in inflamed, irritated, itchy, and reddened eyelids. A number of diseases and conditions can lead to blepharitis. It can be caused by the oil glands at the base of the eyelashes becoming clogged, a bacterial infectionallergies, or other conditions.

The severity and course can vary. Onset can be acute, resolving without treatment within 2–4 weeks (this can be greatly reduced with lid hygiene), but more generally is a long-standing chronic inflammation of varying severity.

It may be classified as seborrhoeic, staphylococcal, mixed, posterior or meibomitis, or parasitic.  It usually does not cause permanent damage.